Helping the endangered Murray Spiny Crayfish

The Murray Spiny Crayfish (Euastacus armatus) is the second largest freshwater crayfish in the world. No wonder it's considered a popular catch amongst the fishing community. Unfortunately though, the species has experienced a significant decline in its abundance and distribution in the last 50 years and is now listed as threatened in Victoria.     

Murray Spiny Crayfish are found in a wide range of habitats from small upland streams to large lowland rivers. They prefer clean, fast-flowing water with abundant and complex structural habitat. In Victoria, you will often find them in the Mitta Mitta, Ovens, and Goulburn River catchments.  

Murray Spiny Crayfish live for over 25 years. They're slow growing, have limited mobility, low reproductive output and a maturation age between 8-9 years. This makes them particularly vulnerable to blackwater events which results in extremely low levels of oxygen in the water.

The Victorian Fisheries Authority (VFA) have regulations specific to the Murray Spiny Crayfish including a:  

• restricted harvest season (July 1st to August 31st)

• restricted harvest zone (Murray River and tributaries below Hume Weir downstream to Tocumwal)

• daily catch limit (2 x per person)

• possession limit (4 x per person)

• harvestable slot limit length (100 to 120mm)

• ban on possessing egg bearing females or females with young attached.

There are also regulations for how Murray Spiny Crayfish can be collected. You can read these in the Victorian Recreational Fishing Guide here.

Researchers from the Arthur Rylah Institute (ARI) at the Department of Energy, Environment and Climate Action (DEECA) developed a population model to examine the risks of harvest pressure and blackwater events. The model established that the risk of decline in Murray Spiny Crayfish would increase if an increase in fishing was combined with low frequencies of blackwater disturbance. 

Further work to improve the assessment of the risk of harvest pressure on the Murray Spiny Crayfish has also led to the development of a new and enhanced indicator of female reproductive state called ‘Size at Functional Reproduction’ (SFR). SFR is based on the relationship between the size of a female and the likelihood of the presence of eggs.

When compared with the currently used measurement ‘Size at Onset of Maturity’ (SOM) it was found that SOM underestimated the risk of harvest pressure. The difference between the two indicators was more pronounced with increasing harvest pressure. This indicates that SOM is inadequate to characterise risk.

A large discrepancy in sex ratio (2:1) within catch data was also found. In other words, females may be easier to catch than males because there are twice as many females than males. This suggests that sex ratio as a measure of exploitation and recovery is problematic since it may not adequately reflect the effects of harvest pressure. For this reason, it was also recommended that all females should be protected from harvest.  

Creating a model that takes into consideration both fishing and conservation allows both aspects to be integrated into a flexible risk framework for this species.

Importantly, fisher expectation can be better managed by assessing the impact on harvest of potential changes to fishing regulations. The new SFR also allows the rapid assessment of population reproductive output to help set regional harvest regulations – an approach which is easily transferable to other threatened, recreationally fished species.

This work has been a collaboration between ARI scientists, the Victorian Fisheries Authority, Catchment Management Authorities, and Traditional Owners. 

Dr Scott Raymond from ARI says that further research is currently being undertaken to better understand the biology and ecology of Murray Spiny Crayfish.

“This will enhance our knowledge to better support the management and conservation of this iconic species.” 

Research currently underway to benefit population management strategies for the Murray Spiny Crayfish includes:  

• identifying relationships between environmental variables (e.g., flows, temperature) and abundance and distribution  

• changes in population structure over time 

• movement patterns, recruitment and survivorship 

• handling effects 

• the collection of ecological and biological data to strengthen population models.